Mixer circuits are utilized in a variety of types of integrated circuits to combine signals in many electronic applications. For consumer electronics applications it is always desirable to minimize the mixer circuit cost as well as improve the circuits overall efficiency. It is known that in certain circumstances when the utilizing such circuits there are undesired spurs or signals that are produced that can affect the overall performance of the mixer.
FIG. 1 is a diagram a mixer circuit 100 that may produce these undesirable spurs or signals environment. As is seen, the circuit 100 includes a mixer 102 which is coupled to a power amplifier (PA) driver 104. The PA driver 104 is in turn coupled to a power amplifier (PA) 106. As seen a baseband (BB) signal is provided to the mixer 102 in conjunction with the local oscillator (LO) signal to provide an output signal of fLO+fBB.
As is seen there is also an undesired spur at fLO−3fBB. This undesired spur can become an issue when transmitting data in certain frequency bands under certain standards. For example, under the Long Term Evolution (LTE) telecommunication standard, the LTE band 13 operation can be affected by these spurs as they can fall in the public safety band. Accordingly, it is desirable to remove the inter-modulation signal shown as IM3 to ensure proper operation of a device that utilizes a mixer circuit. It is known that the IM3 signal can be removed in a variety of ways. One way to address this issue is to couple a band pass filter 202 between the mixer 102′ and the PA driver 104′ as shown in FIG. 2. This does reduce the IM3 signal however at a cost of complexity and increased cost because the band pass filter 202 adds to the overall size of the circuit and can significantly increase the chip area and power consumption.
Another way to address this issue is to couple a saw filter 302 between the PA driver 104″ and the PA 106″ as shown in FIG. 3. In this solution, as is seen the saw filter 302 also removes or reduces the undesired spur. However the addition of the saw filter 302 requires more packaging area and is relatively more expensive and power consumption.
Another way to address this issue is to replace the mixer 102 with an active harmonic rejection mixer (HRM) 402 as shown in FIG. 4. Although the active HRM 402 does not have the problems associated with the other solutions above it still has problems in certain environments. To describe these issues in more detail refer now to the following description in conjunction with the accompanying figures.
FIG. 5A is a circuit schematic of a conventional active harmonic rejection mixer 500. The active harmonic rejection mixer 500 includes three mixer elements 502, 504 and 506 coupled in parallel. Each of the mixer elements 502, 504 and 506 each received differential input signals. Each of the mixer elements 502-506 transmits signals with different phases such that the vector sum of the undesirable harmonics (in this example, the 3rd and the 5th) is zero. Mixer element 502 receives an in phase LO signal. Mixer element 504 receives a LO signal that is 45° out of phase with the signals received at mixer element 502. Mixer element 506 receives a LO signal that is 90° out of phase with the signal received at mixer element 502. In this circuit 500, the even order harmonics are rejected due to the differential operation of the circuit. The third and fifth harmonics as illustrated in FIG. 5B are cancelled by the output vectors of the mixer paths and by sizing the transistors in the mixer elements 502-506 in an appropriate manner. Accordingly, the signal for the fundamental harmonic is unattenuated. By contrast the third order harmonic and the fifth order harmonic signals are zero. For example as is seen, the transistors in mixer element 504 are larger than in the mixer elements 502 and 506.
The system requires unwanted signal (harmonics) subtraction, cancellation or rejection of multi-paths. In this type of system, a mismatch in multi-paths (X1, X2 and X3) results in residual error in subtraction, and sets a rejection limitation. Accordingly, the problem with the active HRM 500 is that it has limited linearity, requires high-power and utilizes a large area.
Another type of conventional mixer is a passive voltage sampling mixer. FIG. 5C is a diagram of a conventional passive voltage sampling mixer 550. The mixer 550 includes receives differential I and Q signals at their inputs 552a-552d. In this embodiment, the differential I signals 552a and 552b are separated by 180° and the differential Q signals 552c and 552d are separated by 180°. The outputs of the signals are coupled to an amplifier 554. The LO clocks 552a-552d are non-overlapping and are provided utilizing a 25% duty cycle as shown in FIG. 5D. The circuit 550 does have high linearity, has negligible power consumption and utilizes a small area on a chip or package; however it still has the undesirable spur component.
Accordingly, what is desired is to provide a system and method that overcomes the above issues. The system should be simple, cost effective, easily implemented and adaptable to existing environments. The present invention addresses such a need.